4 Load Balancing The maximum paths range from 1 to 6By default most IP routing protocols install 4 routes in parallelStatic Routes always install 6 routesRip can only load balance paths that have same number of hopsIGRP can load balance up to 6 unequal pathsThe maximum-paths maximum command allows the number of parallel paths used to load balance to be changed in configuration mode

5 Load Balancing (switching)process switching (packets)The router alternates paths on a per packet basisfast switching (per destination)All packets in the packet stream bound for a specific host take the same pathPackets bound for a second host on the same destination network would all take an alternate path

7 How to Solve Routing LoopsCount to InfinitySplit HorizonRoute PoisoningTriggered UpdatesHold Down Timers

8 Count to InfinityDistance Vector Protocols define Infinity as a specific numberLooping continues until Infinity (16 for RIP) is reachedWhen infinity is reached the Network is considered unreachableIn our example the loop would continue until the county reached 16 and then network 1 would be marked unreachable

9 Split HorizonA routing loop occurs when incorrect information is sent to a router that just sent out correct informationThe solution is to avoid sending information back in the direction it cameIn our example Routers A and E would never have received C’s bad info and thus no routing loop would have occurred.

10 Route PoisoningRoute poisoning is used to overcome loops in large networks by setting the hop count to one more than the maximumThe poison reverse rule states “Once you learn of a route through an interface, advertise it as unreachable back through that same interface”.Route poisoning is essentially the same as poison reverse + split horizon

11 Triggered UpdatesBy sending updates faster than the default update convergence occurs must fasterA topology change quickly propagates through a networkTriggered updates used with route poisoning marks a “down” network as unreachable quickly through the network

12 Hold Down TimersWhen a router receives information that a previously accessible route is inaccessible it starts a hold down timer and marks the route as inaccessibleIf an update is received indicating the route is back up before the hold down timer expires then the route is marked accessible againIf an update is received from a different neighbor with a better metric then the route is marked accessible and the hold down time is removedIf an update is received from a different neighbor with a poorer metric before the hold down timer expires then the update is IGNORED

13 Examining Routing TablesShow ip route commandHow did I receive a route?What is the metric?What is the administrative distance?Is the route directly connected?What is the output interface to get to a route?

15 Administrative DistanceAdministrative distance is a default value assigned to each routing protocol that will favor the best route from one protocol over another oneDistances for common protocols are:Directly ConnectedStatic Routes1IGRP100OSPF110RIP120

16 Gateway of Last ResortRouters try to keep routing tables as small as possibleA router may not be able to match a destination network with an entry in it’s routing tableDefault routes can be entered statically or learned dynamicallyip default network x.x.x.x establishes a default route in networks using dynamic routingAny router set with default network x.x.x.x that has a route to x.x.x.x flags x.x.x.x as a candidate default routeA default route can be statically set by saying:ip route {next hop ip or exit interface}If no path to the destination network is found in the routing table then the quad zero default is used

17 Integrating Static Routes with RIPIf your static route was not defined with a network command then it is not distributed unless you use the redistribute static commandip route dest. mask ip/interface admin_distancePackets bound for specific destination networks can be forced to follow a certain pathUsing a higher administrative distance can provide a backup path in case of main link failure

18 RIP Basics To configure use route rip, then network x.x.x.xThe metric is hop countA hop count of 16 is infinityPeriod updates are sent every 30 secondsIt is a distance vector protocolThe entire routing table is sent during updatesThe administrative distance is 120The hold down timer default is 180 seconds

19 IGRP BasicsTo configure use route igrp as-number, then network x.x.x.xThe metrics are bandwidth, delay, load, and reliabilityPeriod updates are sent every 90 secondsThe hold down time is 3x the update timer or 90 secs. x seconds = 280 seconds.It is a distance vector protocolThe entire routing table is sent during updatesThe administrative distance is 100Scalable for very large networks

20 Troubleshooting Routing ProtocolsIs the routing protocol set?show ip protocolsIs the route in the routing table?show ip routeAre the interfaces configured?show running-configAm I using the same version of RIP throughout my network?ping, traceroute, debug